This invention relates to image processing and, more particularly, to a color filter array and method for performing color interpolation.
A digital camera captures an image using a sensor including a large number of pixels, or picture elements. Each pixel may include a light-sensitive photocell or other circuitry that produces a voltage upon receiving incident light. The voltage is converted to digital form in the camera. The digitized image data may be stored, manipulated, or transmitted to another location within or outside the camera.
Rather than recording color, the circuitry of the pixel records intensity information of the image. Accordingly, the color information may be extracted from the intensity data using color filters. Although other color combinations are sometimes used, some arrangements of color filters extract the three primary colors: red, green, and blue. From combinations of the three colors, the entire color spectrum, from black to white, may be derived.
Cameras employ different mechanisms for obtaining the three primary colors from the incoming photons of light. Very high quality cameras, for example, may employ three separate sensors, a first with a red filter, a second with a blue filter, and a third with a green filter. Such cameras typically have one or more beam splitters that send the light to the different color sensors. All sensor photocells receive intensity information simultaneously. Because each photocell is receiving light filtered through a distinct color, each pixel value is dedicated to a single color. The additional hardware, however, makes these cameras relatively expensive.
A second method for recording the color information is to rotate a three-color filter across the sensor. Each pixel may store all three colors, in sequence. However, each color is stored at a different point in time. Thus, this method works well with still, but not candid or handheld photography, because the three colors are not obtained at precisely the same moment.
A third method for recording the three primary colors from a single image is to dedicate each pixel to a different color value. Different photocells of the sensor are filtered with one of the red, green, and blue filters. The arrangement of differently colored filters upon a sensor is known as a color filter array (CFA). CFA sensors allow each of the red, green, and blue pixels to receive image information simultaneously. Once the image is recorded, the true color, or three-color representation, for each pixel may subsequently be derived using color interpolation.
Color interpolation depends on the pattern, or “mosaic,” that describes the layout of the filters on the pixels of the sensor. One common mosaic is known as a Bayer pattern. The Bayer pattern alternates red and green pixels in a first row of the sensor with green and blue pixels in a second row of the sensor. Thus, there are twice as many green pixels (50%) as either red pixels (25%) or blue pixels (25%). The green pixels are preferred because the human eye is more sensitive to luminance in the green color region.
CFA sensors, including the Bayer-patterned sensor, are useful for some color imaging because a single sensor is used, yet all the color information is recorded at the same moment. This allows for smaller, cheaper, and more versatile cameras. Since each pixel records information from one color, information for the remaining colors is missing from that pixel. Accordingly, color interpolation is typically performed on image data produced by a CFA sensor.
A variety of color interpolation algorithms, both adaptive and non-adaptive, may be performed to synthesize the color pixels. Non-adaptive algorithms are performed in a fixed pattern for every pixel in a group. Such algorithms include nearest neighbor replication, bilinear interpolation, cubic convolution, and smooth hue transition.
Adaptive algorithms detect local spatial features in a group of pixels, then apply some function, or predictor, based on the features. Examples of adaptive algorithms include edge sensing interpolation, pattern recognition, and pattern matching interpolation, to name a few.
While CFA patterns are popular in digital camera sensors, performing color interpolation on CFA image data produces mixed results. Since color interpolation provides a more complete representation of the image than the image data recorded by the CFA, a CFA pattern that simplifies the color interpolation algorithm may increase the usefulness of some digital cameras.
Thus, there is a continuing need to provide a color filter array pattern that facilitates efficient color interpolation.